[vorpalhex]

In general yes, but if you say, start using your read data on your write side, then you're in for a very bad time. Likewise you are going to have some very serious timing woes if you violate your domains.

It's easy to read this and go "of course" for a small conceptual system, but in practice even really experienced engineers want to break those rules in production systems - and it's a lot easier to break those rules than fix the system to respect them.

[stingraycharles]

In this specific case I'm talking about e.g. returning errors in case command validation failed. Officially it's supposed to be returned asynchronously through events (and then correlate by the command identifier), but it can be much more pragmatic (and reliable even) to just return an error from the command handler in these cases.

[vorpalhex]

Your writes are always going to return a write model, and that won't account for indexing problems when you don't return an error - so your clients may naively equate a non-error response as a success. If you're the one writing the client, maybe that's not a problem. If you have several consumers who don't know the ins and outs of your system, that might be a very serious problem.

This is the problem with CQRS. The answer to so many of these little tweaks depends on a ton of system knowledge and contracts and promises, and it's really really easy to make the wrong choice.